Protein phosphatase 2A (PP2A) is the most abundant serine/threonine phosphatase in mammalian cells and is important in a number of cellular processes such as cell division, development, and signal transduction. It is also involved in carcinogenesis and forms complexes with the small DNA tumor viruses SV40 and polyoma. It is composed of three subunits; A,B, and C. We have proposed a model of the structure of the A subunit based on its amino acid sequence which contains 15 repeats of 38-43 amino acids each of which is predicted to form 2 amphipathic alpa helices. We seek sufficient SR time to collect data on several promising heavy atom derivatives using xenon pressure as well.